This proposal continues our characterization of the role of neurotrophic signaling pathways in the neural plasticity induced in the ventral tegmental area (VTA)-nucleus accumbens (NAc) reward circuit by chronic exposure to opiate or stimulant drugs of abuse. The proposal has three Aims: 1) to delineate the cell-type specific actions of BDNF-TrkB signaling in the VTA-NAc in regulating molecular, cellular, and behavioral responses to opiates and cocaine; 2) to delineate the adaptations in BDNF-TrkB signaling cascades that mediate the ability of opiates to induce morphological changes in VTA dopamine neurons; and 3) to delineate the adaptations in BDNF-TrkB signaling cascades that mediate the ability of cocaine and opiates to induce opposite morphological changes in NAc medium spiny neurons. We have made important progress in each of these Aims over the past 5 years, and have substantial preliminary data to support highly penetrating and mechanistic studies over the proposed new grant period. Using genetic mutant mice and viral-mediated gene transfer, which has enabled highly localized knockouts of BDNF or TrkB within the VTA or NAc of adult animals, we have demonstrated distinct roles for BDNF-TrkB signaling in regulating responses to opiates and cocaine. Using more advanced tools, which now enable such knockouts in specific neuronal cell types within these brain regions, we will characterize the specific influence of D1 vs. D2 containing NAc medium spiny neurons, and VTA dopamine neurons, in these phenomena. One of the most dramatic changes that opiates induce in VTA dopamine neurons is a decrease in their overall size. This decrease is mediated by an impairment in BDNF signaling within these neurons, specifically, downregulation of the IRS-AKT pathway. We now have evidence that this downregulation is part of overall opiate-induced adaptations in AKT-RHEB-mTOR signaling. The proposed experiments are designed to further establish the detailed molecular events that underlie this morphological adaptation. In contrast, cocaine and opiates induce distinct changes in NAc medium spiny neurons: cocaine increases, whereas opiates decrease, the neurons' dendritic arborizations and spine density. We have shown that these adaptations too involve altered BDNF signaling, and now propose to further characterize the precise changes in neurotrophic signaling pathways, including changes in NF?B signaling that mediate these adaptations. Together, the proposed experiments will provide fundamentally new insight into the dramatic ways in which opiates and stimulants change VTA-NAc neurons, information which could be mined in future years to define improved diagnostic tests and treatments for drug addiction.